The senators wrote that because there are no federally mandated warning labels for e-cigarettes, the companies are producing their own warning labels that fail to list all of the health risks. Further, the senators write, the proposed FDA warning also does not go far enough. The FDA’s proposed label reads “WARNING: This product contains nicotine derived from tobacco. Nicotine is an addictive chemical,” the letter stated. “We support requiring a label on nicotine’s addictive properties, but we ask the FDA pursue requirements for more extensive warnings that address health risks that e-cigarettes pose,” The Hill reports.

Sen. Boxer’s office issued a news release with the full text of the letter. The letter says that the lack of uniformity of warnings across companies highlights “how voluntary health warnings often leave out known dangers from nicotine use – such as risks to adolescent brain development and pregnant women, as well as the dangers posed by additives and other chemicals that may be in e-cigarettes, such as benzene and formaldehyde.”

Liquid nicotine can be toxic, and the Centers for Disease Control and Prevention (CDC) has reported a sharp increase in nicotine poisoning incidents. For young children, even a small amount of liquid nicotine can be fatal. The liquid comes in fruit and candy flavors that appeal to children and manufacturers are not required to use childproof bottles. The FDA says it has received reports of breathing difficulties, chest pain and other cardiovascular problems, and allergic reactions from e-cigarette users. Recent studies have shown that some e-cigarettes can get hot enough to produce some of the same carcinogens as conventional cigarettes, The New York Times reports.